Thin film transistors (“TFT's”) are widely used as switching elements in electronics, for example, in active-matrix liquid-crystal displays, smart cards, and a variety of other electronic devices and components thereof. The thin film transistor (TFT) is an example of a field effect transistor (“FET”). The best-known example of an FET is the “MOSFET (Metal-Oxide-Semiconductor-FET) that can be used for high-speed applications. Most thin film devices are made using amorphous silicon as the semiconductor because amorphous silicon is a less expensive alternative to crystalline silicon. This fact is especially important for reducing the cost of transistors in large-area applications. Application of amorphous silicon is limited to low speed devices, however, since its maximum mobility (0.5-1.0 cm2/V·sec) is about a thousand times smaller than that of crystalline silicon.
The use of amorphous silicon has its drawbacks. The deposition of amorphous silicon, during the manufacture of transistors, requires relatively costly processes, such as plasma enhanced chemical vapor deposition and high temperatures (about 360° C.) to achieve the electrical characteristics sufficient for display applications. Such high processing temperatures disallow the use of substrates, for deposition, made of certain plastics that might otherwise be desirable for use in applications such as flexible displays.
More recently, organic materials have received attention as a potential alternative to amorphous silicon for use in semiconductor channels of TFT's. Organic semiconductor materials are simpler to process, especially those that are soluble in organic solvents and, therefore, capable of being applied to large areas by far less expensive processes, such as spin-coating, dip-coating and microcontact printing. Furthermore, organic materials may be deposited at lower temperatures, opening up a wider range of substrate materials, including plastics, for flexible electronic devices. Accordingly, thin film transistors made of organic materials can be viewed as a potential key technology for plastic circuitry or devices where ease of fabrication or moderate operating temperatures are important considerations or mechanical flexibility of the product is desired.
Organic semiconductor materials can be used in TFT's to provide the switching or logic elements in electronic components, many of which require significant mobilities, well above 0.01 cm2/V·sec, and current on/off ratios (hereinafter referred to as “on/off ratios”) greater than 1000. Organic TFT's having such properties are capable of use for electronic applications such as pixel drivers for displays, identification tags, portable computers, pagers, memory elements in transaction carts, and electronic signs. Organic materials for use as potential semiconductor channels in TFTs are disclosed, for example, in U.S. Pat. No. 5,347,144 (Garnier et al.).
Considerable efforts have been made to discover new organic semiconductor materials that can be used in FET's to provide switching or logic elements in electronic components, many of which require significant mobilities well above 0.01 cm2/V·sec, and current on/off ratios (hereinafter referred to as “on/off ratios”) greater than 1000. Organic FETs (“OFET's”) having such properties can be used for electronic applications such as pixel drivers for displays and identification tags. Most of the compounds exhibiting these desirable properties are “p-type” or “p-channel,” however, meaning that negative gate voltages, relative to the source voltage, are applied to induce positive charges (holes) in the channel region of the device.
As an alternative to p-type organic semiconductor materials, n-type organic semiconductor materials can be used in FET's where the terminology “n-type” or “n-channel” indicates that positive gate voltages, relative to the source voltage, are applied to induce negative charges in the channel region of the device.
Moreover, one important type of FET circuit, known as a complementary circuit, requires an n-type semiconductor material in addition to a p-type semiconductor material. In particular, the fabrication of complementary circuits requires at least one p-channel FET and at least one n-channel FET. Simple components such as inverters have been realized using complementary circuit architecture. Advantages of complementary circuits, relative to ordinary FET circuits, include lower power dissipation, longer lifetime, and better tolerance of noise. In such complementary circuits, it is often desirable to have the mobility and the on/off ratio of an n-channel device similar in magnitude to the mobility and the on/off ratio of a p-channel device. Hybrid complementary circuits using an organic p-type semiconductor and an inorganic n-type semiconductor are known, but for ease of fabrication, an organic n-channel semiconductor material would be desired in such circuits.
Only a limited number of organic materials have been developed for use as a semiconductor n-channel in OFET's. One such material, buckminsterfullerene C60, exhibits a mobility of 0.08 cm2/V·sec but it is considered unstable in air (Haddon et al. Appl. Phys. Let. 1995, 67, 121). Perfluorinated copper phthalocyanine has a mobility of 0.03 cm2/V·sec and is generally stable to air operation, but substrates must be heated to temperatures above 100° C. in order to maximize the mobility in this material (Bao et al. Am. Chem, Soc. 1998, 120, 207). Other n-channel semiconductors, including some based on a naphthalene framework, have also been reported, but with lower mobilities. One such naphthalene-based n-channel semiconductor material, tetracyanonaphthoquino-dimethane (TCNNQD), is capable of operation in air, but the material has displayed a low on/off ratio and is also difficult to prepare and purify.
Aromatic tetracarboxylic diimides, based on a naphthalene aromatic framework, have also been demonstrated to provide, as an n-type semiconductor, n-channel mobilities up to 0.16 cm2/V·sec using top-contact configured devices where the source and drain electrodes are on top of the semiconductor. Comparable results could be obtained with bottom contact devices, that is, where the source and drain electrodes are underneath the semiconductor, but a thiol underlayer must then be applied between the electrodes (that must be gold) and the semiconductor as described in U.S. Pat. No. 6,387,727 (Katz et al.). In the absence of the thiol underlayer, the mobility of these compounds was found to be orders of magnitude lower in bottom-contact devices. This patent also discloses fused-ring tetracarboxylic diimide compounds, one example of which is N,N′-bis(4-trifluoromethyl benzyl)naphthalene diimide. The highest mobilities of 0.1 to 0.2 cm2/V·sec were reported for N,N′-dioctyl naphthalene diimide.
In a different study, using pulse-radiolysis time-resolved microwave conductivity measurements, relatively high mobilities have been measured in films of naphthalene diimides having linear alkyl side chains (Struijk et al., J. Am. Chem. Soc. Vol. 2000, 122, 11057).
U.S. Patent Application Publication 2002/0164835 (Dimitrakopoulos et al.) discloses n-channel semiconductor films made from perylene diimide compounds, as compared to naphthalene-based compounds, one example of which is N,N′-di(n−1H,1H-perfluorooctyl) perylene diimide. Substituents attached to the imide nitrogens in the diimide structure comprise alkyl chains, electron deficient alkyl groups, and electron deficient benzyl groups, and the chains preferably having a length of four to eighteen atoms. Devices based on materials having a perylene framework used as the organic semiconductor have low mobilities, for example 10−5 cm2/V·sec for perylene tetracarboxylic dianhydride (PTCDA) and 1.5×10−5 cm2/V·sec for N,N′-diphenyl perylene diimide (PTCDI-Ph) (Horowitz et al. Adv. Mater. 1996, 8, 242 and Ostrick et al. J. Appl. Phys. 1997, 81, 6804).
The morphology of an organic film has a strong impact on the charge transport and overall device performance of organic thin film transistors. In general, the morphology of organic films depends directly on the chemical structure of the molecules that controls the interaction between the molecules. In crystalline organic films defects, like grain boundaries and disorder inside the grains, are limiting factors for the mobility and the diffusion length of the charge carriers. The extent of π-stacking between the molecules determines whether the organic film will be highly crystalline or totally amorphous independently of other growth controlling parameters like the substrate and its temperature.
In perylene and naphthalene diimide based OFET's, many experimental studies have demonstrated that morphology of the thin film has strong impact on the device performances. Theoretical calculation and experimental characterization (particularly X-ray diffraction), have shown that the molecular packing in PDI is very sensitive to the side chains (Kazmaier et al. J. Am. Chem. Soc. 1994, 116, 9684). In perylene diimide based n-channel OFET devices, changing the side chain from n-pentyl to n-octyl increases the field effect mobility of from 0.055 cm2/V·sec to 1.3 cm2/V·sec, respectively (Chesterfield et al. J. Phys. Chem. B 2004, 108, 19281). Such sensitivity to the type of side-chain is a manifestation of an aggregation effect and it provides potentially an effective way to control and optimize the molecular packing for enhanced π-orbital overlap between neighboring molecules, a necessary for efficient carrier transport. U.S. Pat. No. 7,422,777 (Shukla et al.) discloses N,N′-dicycloalkyl-substituted naphthalene diimide compounds, which in thin films, exhibit optimum packing and exhibit n-channel mobility up to 6 cm2/V·sec in OFET's. U.S. Pat. No. 7,579,619 (Shukla et al.) discloses N,N′-di(arylalkyl) substituted naphthalene diimide compounds that exhibit high n-channel mobility up to 3 cm2/V·sec in top-contact OFET's.
U.S. Patent Application Publications 2008/0135833 (Shukla et al.) and 2009/0256137 (Shukla et al.) describe n-type semiconductor materials for thin film transistors that include configurationally controlled N,N′-dicycloalkyl-substituted naphthalene 1,4,5,8-bis-carboximide compounds or N,N′-1,4,5,8-naphthalenetetracarboxylic acid imides having a fluorinated substituent, respectively.
As discussed above, a variety of naphthalene diimides have been made and tested for n-type semiconducting properties. In general, these materials, as an n-type semiconductor, have provided n-channel mobilities up to 6 cm2/V·sec using top-contact configured devices. However, besides charge mobility, improved stability and integrity of the semiconductor layer is an important goal. A way to improve organic semiconductor layer stability and integrity in a device would be to include the organic semiconductor molecule in a polymeric additive. However, the performance of OFET's, characterized by parameters such as the field effect mobility and threshold voltage, depends in part upon the molecular structure and crystalline order of the semiconductor film. Structure and molecular ordering of the semiconductor film depends in turn on how the thin film is deposited. It is generally believed that increasing the amount of molecular order by increasing crystal size, reducing the density of crystalline defects, or improving short-range molecular order, permits charge carriers, that is, electrons or holes, to more efficiently move between molecules. This can increase the field effect mobility.
Advantageous molecular order that gives high field effect mobility can be achieved using some relatively expensive deposition techniques. In contrast, deposition techniques that enable inexpensive production or production of films of a desired uniformity and thickness can produce films that exhibit relatively small field-effect mobility. For example, a solvent cast film that is permitted to slowly dry often exhibits relatively high field effect mobility when incorporated into an OFET. Unfortunately, some deposition techniques that are more amenable to manufacturing do not readily permit slow evaporation of solvent. For example, though spin coating can yield relatively uniform thin films, the solvent usually leaves the film relatively quickly, generally leading to a low degree of crystal order. Field-effect mobility, for example, can be a factor of about 10 to 100 smaller than for cast films. Other manufacturing processes such as screen printing or various thin-film coating methods may yield desirable film morphology, but not a desirable molecular order.
The addition of polymeric additive in solvent cast semiconductor films could solve some of the aforementioned problems. The addition of a polymer in coating solvent could provide a better control over the semiconductor film morphology by acting as a plasticizer during thermal annealing process. Furthermore, if an added polymer phase segregates away from the semiconductor-dielectric interface it could also increase the ambient operational stability of devices. However, the addition of a polymer could disrupt the molecular ordering in the semiconducting layer that could leads to disruption of the orbital overlap between molecules in the immediate vicinity of the gate insulator leading to reduced mobility. Electrons or holes are then forced to extend their path into the bulk of the organic semiconductor, which is an undesirable result. Certain organic semiconducting materials are expected to be more susceptible than others to the effects of added polymer.
EP 910,100 (Hsieh) describes compositions for conductive coatings comprising a polymer binder, charge transport molecules, and an oxidant that is used to increase carrier concentration. Such coatings may be useful as conductive electrodes for electronic devices, such as source-drain and gate electrodes in FET's.
U.S. Pat. No. 5,500,537 (Tsumura et al.) describes FET's with at least two different organic channel materials, both of which are semiconductors. The application also mentions that a further “electrically insulating material” can be mixed in but does not teach what such material may be or how it is applied.
EP 478,380 (Miura et al.) describes organic thin films comprising of mixed charge-transfer complexes of donor and acceptor materials. The complex thin film can be affected to change its state from neutral to ionic by the application of an electric field. When used in a FET, the transition leads to an abrupt change in the carrier density in the channel. Multi-stack channels are also described using several double-layers of a charge transfer complex layer followed by an insulating poly(vinylidene fluoride) (PVDF) layer. The insulating PVDF layer is not used as a binder.
U.S. Pat. No. 5,625,199 (Baumbach et al.) discloses complementary circuits with p- and n-type OFET's. It also mentions that p-type organic material may be made of p,p′-diaminobisphenyl in polymer matrices. However, there is no teaching of the use of polymer matrices with n-type organic materials or what polymer matrix may be and does not disclose any other compound other than p,p′-diaminobisphenyl.
U.S. Pat. No. 3,963,498 (Trevoy) discloses amine salts of linear polyanilines as useful semiconductors. It further discloses that an organic binder may be added to the amine salt.
Certain low polarity polymeric additives are described in U.S. Pat. No. 7,095,044 (Brown et al.) for use with organic semiconductors in FET's. This patent discloses an OFET structure in which the semiconductor layer comprises an organic semiconductor and a polymeric binder that has an inherent conductivity of less than 10−6 S cm−1 and a permittivity at 1,000 Hz of less than 3.3. The patent does not specifically teach how to use polymer binders with n-type materials, such as naphthalene diimide based n-type semiconductor materials.
WO 2004/057688 (Veres et al.) discloses compositions for use as organic semiconducting materials in which a low molecular weight semiconducting material is mixed with higher molecular weight organic semiconducting material. One composition comprises at least one higher molecular weight organic semiconducting compound having a number average molecular weight (Mn) of at least 5000 and at least one lower molecular weight compound having a number average molecular weight (Mn) of 1000 or less. The polymeric binder is a semiconductor as well. However, there is no teaching about how to use n-type organic semiconducting materials, such as naphthalene diimides, with polymeric binders.
U.S. Pat. No. 7,576,208 (Brown et al.) describes the use of low polarity binders in polyacene-based semiconductors.
U.S. Pat. No. 7,586,080 (Chabinyc et al.) discloses a layered structure comprising a carrier-transporting substructure. The carrier-transporting substructure includes at least one of polymer material, blends of polymers, or polymerized organic compounds. Furthermore, the carrier-transporting substructure includes one or more of titanyl phthalocyanine, poly(vinyl butyral), poly(9,9-dioctylfluorene-alt-benzothiadiazole) (F8BT), N,N′-diphenyl-N,N′bis(3-methylphenyl)-(1-1′-biphenyl)-4,4′-diamine (TPD), and alkylated-4,-4′diphenoquinones (DPQ).
U.S. Pat. No. 7,244,960 (Spreitzer et al.) discloses solutions of organic semiconductors containing one or more additives that can be organic compounds containing the heteroatom silicon, germanium, or fluorine, or an amphiphile and wherein at least one additive is an organic siloxane-containing compound.
As described above, most of the semiconductor layer compositions described in the art are p-type and similar n-type semiconductor layer formulations are not well known. As development of advanced circuits (for example, complementary circuits) requires both p- and n-type semiconductors there is a need to develop n-type semiconductor layer formulations. There is a need for n-type semiconducting layer composition that is easily processed and provides control over the film morphology. An additional problem in development of robust n-type semiconductors is their operational stability. In contrast to p-type materials, device performance of n-type semiconductors is degraded in the presence of atmospheric oxidants such as O2 and H2O. Electronically this susceptibility manifests itself in the formation of efficient electron traps or the sharp degradation of carrier mobility upon exposure to air. Another need is to develop n-type layer composition that is less susceptible to the ambient and minimizes the deterioration of electronic properties over time.